Two groups of patients undergoing open-heart surgery were given prophylatic courses of antibiotic lasting 5 days. One group (61 patients) received a cephalosporin and the 2nd (57 patients) received a combination of penicillin, flucloxacillin and streptomycin. The overall major infection rate was low (3-4%), particularly so in the cephalosporin group (1.6%). There was no increased nephrotoxic effect of the cephalosporin; any nephrotoxic effect that was present was temporary and clinically unimportant. The major infecting organism in both groups was Staphylococcus epidermidis. The efficiency of any prophylactic regime which omits gentamicin, to which epidermidis is usually sensitive, remains in doubt.